Aiming For The Ships P

By the time P67 took over, the command module had slowed below the velocity required for orbit. The job then for P67 was to continue controlling the direction of the lift vector and the g-forces thus generated, rolling the spacecraft this way and that as required, to guide it to its planned impact point while the speed decreased to only 300 metres per second. As well as controlling how far the spacecraft would fly by aiming the lift vector up or down, P67 also compensated for whether the trajectory was taking the spacecraft to either side of the target. By rolling up to 15 degrees to either side, a useful amount of lift was aimed to the left or right without excessively affecting the lift in its up/down axis. During P67, the DSKY's display showed how much sideways angle they were steering and how far the computer reckoned their current impact point was from the ideal, both left/right and long/ short.

''Here comes the water again on my feet,'' laughed Stafford on Apollo 10.

''From the freaking tunnel! It's cold, John baby,'' said Stafford.

''It's going to pulse the lift vector up,'' said Stafford.

''Four g's. Going to go lift vector up,'' announced Young.

''We'll let her shoot, lift vector up,'' said Stafford, happy that the computer was doing its job.

''Okay, Houston, we're showing 6 miles short right now,'' called Stafford, ''and we're coming on in; pulling about 4 g's and this machine just flying like crazy. Boy, it's really going.''

''Well, I'll tell you, this thing is beautiful,'' said Young in admiration for what the little module was capable of doing. The computer reckoned they were going to land 11 kilometres short, and had rolled the CM around to a feet-up, lift-up attitude to gain them more distance.

''And we're pulling about 3.5 g's now. We're rolling right 60 degrees, and we're practically on top of the target. EMS is reading 21 miles to go. Okay, we're coming down. I guess we're about 150 K right now.''

They were still 47 kilometres up but most of their horizontal speed was gone. Stafford began thinking about how to keep track of what was increasingly a vertical plummet.

''Steam pressure. Get the steam pressure,'' he called.

Before Apollo 8 flew, Bill Anders had come up with an idea to give the crews a backup means of determining how high they were, rather than just depending on their altimeter. They had a meter that indicated the pressure in the steam duct that led from the evaporator. Since its reading would begin to rise at an altitude of 90,000 feet or 27.4 kilometres, they could start a watch as soon as it came off the zero peg and check the timing of subsequent events from there, particularly the deployment of the parachutes. This was the reason mission control had included a timing for when they could expect 90,000 feet in the entry PAD. The technique worked well when Anders used it on his flight, but subsequent LMPs found it less reliable.

''The steam pressure peg was somewhere between 5 and 10 seconds late,'' explained Ed Mitchell during Apollo 14's debrief. ''At 6:36 it still had not started to move up so I switched to secondary. It hadn't moved either, and I remarked about it to Al. About that time, it started to move.'' Mitchell was concerned that if relied upon, it could be misleading. ''If we had to enter on that and go by times, I think we might have been in trouble.''

Duke found that it was even later than predicted on Apollo 16. ''The only thing that was off nominal was the steam pressure was 32 seconds late. I started watch on the time that steam pressure pegged. If we had to call the times based on that, we'd have been late.''

Once the spacecraft's velocity had slowed to less than 300 metres per second, P67 changed the computer's display to show the range to splashdown and the current latitude and longitude of the spacecraft. By now they were essentially above the landing site, 20 kilometres up and falling more or less vertically. P67's final act was to inhibit further thruster firing by transferring spacecraft control to the SCS before the program was terminated by a press of the 'Proceed' button.

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